1. Field of the Invention
The invention relates to a fuel injection control method. More particularly, the invention relates to a pressure boosting common rail fuel injection apparatus which includes pressure boosting means, and in which a pressure of fuel supplied from a common rail is further increased, and the fuel is injected from a fuel injection valve, and a fuel injection control method therefor.
2. Description of the Related Art
A so-called common rail fuel injection apparatus is generally known, in which high pressure fuel supplied from a fuel pump is stored in a common rail (an accumulator), the fuel is supplied to an in-cylinder fuel injection valve provided in each cylinder of an internal combustion engine so that the fuel is injected directly into a combustion chamber of each cylinder.
In the common rail fuel injection apparatus, a pressure in the common rail can be controlled to a given value. Therefore, a rate of injection from a fuel injection valve can be controlled to an appropriate value according to an engine operating state so that a good combustion state can be maintained in the cylinder, irrespective of the engine operating state.
Particularly, in a direct injection diesel engine, a fuel injection period during which fuel can be injected into the cylinder is limited, depending on a relationship between the fuel injection period and a position of a piston. Therefore, when an engine rotational speed is high while the engine is operated, the period during which fuel can be actually injected may become extremely short, and it may become difficult to supply a large amount of fuel to the cylinder. However, in the common rail fuel injection apparatus, a fuel injection pressure can be maintained at a high value irrespective of an engine rotational speed. Therefore, by using the common rail fuel injection apparatus, it is possible to supply a large amount of fuel into the combustion chamber in a short time so that a high rotational speed and high output can be achieved in the diesel engine.
Meanwhile, since supercharging is generally performed in diesel engines, it is required to further increase the output of the diesel engine by increasing the fuel injection amount and using the supercharging, and accordingly it is required to further intensify the fuel injection pressure so as to further increase an amount of fuel supplied to the cylinder.
However, in the common rail fuel injection apparatus, the fuel injection pressure has been already set to a high value (e.g., approximately 180 MPa) that is close to a limit value. Therefore, in order to further increase the fuel pressure in the common rail, it is necessary to increase a designed pressure of all elements of a fuel injection system such as the fuel pump, the common rail, and delivery pipes. However, it is not practical to increase the designed pressure of all the elements of the fuel injection system, considering problems such as an increase in cost and a decrease in reliability.
Accordingly, in order to solve the problems, a pressure boosting common rail fuel injection apparatus is proposed. In the pressure boosting common rail fuel injection apparatus, a fuel pressure in a common rail is set to a value that is substantially equal to, or lower than a fuel pressure in a common rail in a conventional apparatus, and a pressure boosting device is employed for further increasing the pressure of fuel supplied to a fuel injection valve from the common rail at a portion that is closest as possible to a nozzle hole of the fuel injection valve.
In the pressure boosting common rail fuel injection apparatus, since the pressure boosting device is used for further increasing the pressure of the fuel supplied to the fuel injection valve from the common rail, the actual fuel injection pressure of the fuel injection valve can be set to a higher value (e.g., approximately 250 MPa) by setting only the designed pressure of a portion from the pressure boosting means to the fuel injection valve to a high value while the designed pressure of the elements such as the fuel pump and the common rail is set to a value that is substantially equal to (or lower than) the designed pressure in the conventional apparatus. Therefore, it is possible to boost the fuel injection pressure while suppressing a large increase in the cost.
Published Japanese Translation of PCT application No. JP-T-2002-539372 discloses an example of such a pressure boosting common rail fuel injection apparatus using such a fuel boosting device.
In the pressure boosting common rail fuel injection apparatus disclosed in the Published Japanese Translation of PCT application No. JP-T-2002-539372, a pressure boosting unit including a pressure boosting piston is used as a pressure boosting device. The pressure boosting unit is provided between a common rail and a nozzle hole of a fuel injection valve. The pressure boosting piston is formed by connecting a pressure receiving piston with a large diameter, and a pressurizing piston with a small diameter. The pressure boosting piston pressurizes the fuel according to an area ratio between the pressure receiving piston and the pressurizing piston. That is, in the pressure boosting piston, the fuel pressure in the common rail is applied to the pressure receiving piston with the large diameter, whereby the fuel supplied to a pressurizing chamber from the common rail is pressurized by the pressurizing piston with the small diameter. Thus, the pressure of the fuel supplied to the fuel injection valve can be increased to a value higher than the fuel pressure in the common rail, which is decided according to the area ratio between the piston having the large diameter and the piston having the small diameter.
In the pressure boosting common rail fuel injection apparatus disclosed in the Published Japanese Translation of PCT application No. JP-T-2002-539372, the pressure boosting device increases the pressure of fuel supplied to the fuel injection valve. Accordingly, it is possible to increase the fuel injection pressure without increasing the designed pressure of elements of the entire fuel injection apparatus.
However, in a case where the pressure boosting device is provided so that the fuel injection pressure is increased, if the same fuel injection control as that performed in the conventional common rail fuel injection apparatus is employed, the control becomes complicated.
In the conventional common rail fuel injection apparatus, the volume of the common rail is set so as to be sufficiently large compared to the amount of fuel injected one time, the fuel pressure supplied to the fuel injection valve is constant throughout the fuel injection period (the period during which the fuel injection valve is opened) is constant.
Accordingly, in the conventional common rail fuel injection apparatus, the fuel injection amount is decided based on an engine operating condition (accelerator pedal operation amount (a depression amount of an accelerator pedal) and an engine rotational speed), using a numerical table (map) that is made in advance. Then, required common rail fuel pressure (fuel injection pressure) and injection timing (injection starting timing) are decided based on the decided fuel injection amount and the engine rotational speed, using respective separate maps which are prepared in advance, and in which fuel injection amount and the engine rotational speed are used as parameters. Also, the fuel injection period (the period during which the fuel injection valve is opened) is decided using a map in which the fuel injection amount and the common rail pressure are used as parameters.
Meanwhile, when using the pressure boosting common rail fuel injection apparatus including a boosting piston as disclosed in the Published Japanese Translation of PCT application No. JP-T-2002-539372, it takes certain time until the fuel pressure is increased to a final pressure (boosted pressure) after operation of the boosting device is started, and the fuel pressure is changed (increased) with time until the boosted pressure is reached. Therefore, when the fuel injection period is overlapped with the period during which the pressure is being increased at the time of boosting the pressure, the fuel injection pressure is changed during the fuel injection.
Therefore, even if a target fuel pressure is decided, for example, based on the fuel injection amount and the engine rotational speed, it is necessary to perform an additional complicated calculation considering the increase in the fuel pressure during a period (pressure boosting period) from when operation of the pressure boosting means is started until when the fuel injection is started, and pressure boosting start timing, in order to decide timing at which the actual fuel injection pressure becomes equal to the aforementioned target fuel pressure.
Further, in the case where the fuel injection period is decided using the map in which the fuel injection amount and the common rail pressure are used as parameters according to the conventional method, when the fuel injection pressure is changed (i.e., the rate of injection is changed) during the injection, it is necessary to provide maps in which multiple parameters are used, for example, according to a change pattern of the fuel injection pressure during boosting of the pressure, and the injection starting timing, since the fuel injection period varies depending on a time point at which the fuel pressure used for deciding the fuel injection period is obtained. As a result, the calculation of the fuel injection period becomes complicated.
Accordingly, in the case where the pressure boosting common rail fuel injection apparatus is used, if the fuel injection control is performed using the same map as that used in the conventional method, the control becomes complicated.